This proposal is in response to the RFA-HL-13-027: Functional Assays to Screen Genomic Hits (R21/R33). In the past 15 years over 40 genetic loci have been identified as associated with increased risk for cardiac arrhythmia and/or sudden cardiac death with Mendelian and multigenic inheritance. With the advance of rapid clinical genetic testing and Next-Generation sequencing, the number of mutations/variants has grown to greater than 1000 for the top loci alone (LQT1-3). A large majority of these variants have not been functionally phenotyped however. This poses a considerable dilemma for the clinical geneticist and managing cardiologist-are uncharacterized variants/mutations to be treated as true risks for arrhythmia? This decision is critical because therapy includes life-long medication, behavioral modification and often implantation of cardiac defibrillators. Moreover, these management decisions extend to all other family members harboring the variants. Furthermore, these clinical scenarios will soon become increasingly common as rapid, accessible and affordable whole exome/genome takes its place in the clinical arena. Till now, functional phenotyping of new variants has been performed in piecemeal fashion, with reports of one or a handful of variants at a time. This usually takes place as a side-project for investigators who are pursuing other aspects of cardiac biology. We propose to functionally phenotype all of the disease-associated genetic variants and most of the rare polymorphisms that have appeared in genetic databases for the loci most commonly reported for hereditary arrhythmias. Variants will be created in cDNAs for expression in human cell lines. Functional expression will be assessed with a high-throughput electrophysiology system. Subsequently, those variants exhibiting a loss-of-function phenotype will be further assayed for surface trafficking defects by biochemical analysis. We will create a publically available database with both ongoing and completed results that will allow other investigators to provide input. Creation of this public resource will provide clinical geneticist and managing physicians a valuable resource to fully analyze the results from genetic testing of their patients. Successful competition of this project will provide a framework and resource for investigators in the field and have immediate impact on clinical management of an increasing number of patients. Accordingly we believe that this proposal is in the spirit of both the RFA-HL- 13-027 and the mission of the NIH/NHLBI.